The man who saunters into the Dublin bar has an outsize shamrock in his lapel, a lilting old Irish ditty on his lips and an Oscar Wilde centre parting. He is, in a phrase, a palpable fraud. After calling up a drop of the hard stuff, he sidles over to a pair of customers with even harder faces, and sets about winning their confidence. "Oscar" claims that he's planning a terrorist attack in London, and needs some high explosives - "tis the Albert Hall, at all, and we've run out of the jelly" - but their silence is unwelcoming, so he switches to a different tack. "But why are we talking in the cursed tongue when we have the Gaelic?" And, leaning back with a seraphic smile across his jowly features, he croons "Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch".

They punch him in the nose.

The scene is, as it should be, funnier to watch than to read about, and it has good claims to represent the precise moment when Peter Sellers's film career really got into its stride. It comes from Mario Zampi's The Naked Truth (1958), a comedy written by Michael Pertwee with Sellers's protean abilities in mind. The film is one of those highly enjoyable, plot-driven little features the British cinema used to turn out without apparent effort. Sellers plays the part of Wee Sonny Macgregor, an unctuous game-show host whose stage identity is a hybrid of Wilfred Pickles and Andy Stewart. Offstage, though, Wee Sonny is not merely a lascivious cynic but a slum landlord, exploiting the same pensioners who adore him. When a blackmailer (Dennis Price) threatens to expose his racket, Macgregor fights back with what he believes to be his main weapon - he is the "jack of all faces", and machinates against Price in the personae of a doddering jobsworth, a moronic aristocrat and several other ripe characters.

But - and this is the cream of the jest - Wee Sonny is, unlike Sellers, not a very good impersonator. What makes that Dublin pub scene so amusing is less the idiocy of the performance than the sublime confidence with which it is executed. (Sellers would milk this gag again, particularly for the part of Inspector Clouseau, the dolt who never doubts that he is a genius). When the camera goes close in on Sellers's face, you're witnessing the kind of hopeless incompetence that can only be drawn by supreme competence.

Sellers's performance in The Naked Truth is one of the touchstones that should be kept in mind when watching the three-part Arena on the comedian. Viewers who remember Sellers mainly for the later and wearier Clouseau vehicles (the rot set in somewhere around 1976, with The Pink Panther Strikes Again) may well wonder what all the fuss is about. And even those who are familiar with the earlier films are not always enchanted by what they have seen.

For his admirers, Sellers's career is the story of a prodigious talent that took a wrong turn. Sir Peter Hall, who directed Sellers in his one - disastrous - attempt at stage acting, still considers him "as good an actor as Alec Guinness, as good an actor as Laurence Olivier". But for sceptics, Sellers was essentially a radio comedian of indisputable brilliance whose skills were radically unsuited to the scrutiny of the camera. As David Thomson puts it in his magisterial Biographical Dictionary of the Cinema, "his films have the grinding tedium of a heartless virtuoso... he was a dull character actor, chastened by the medium's distaste for darting in and out of different characters without revealing his own."

Thomson's criticisms are searching, and not easy to qualify. It's certainly close to the truth to say that, with a single exception, Sellers didn't make a film worth remembering after 1964: the latter half of Sellers's filmography can be divided into the decreasingly inspired Pink Panther movies, a couple of badly dated crowd-pleasers (There's a Girl in My Soup), a slew of commercial or critical disasters (Casino Royale, The Bobo, The Magic Christian) and several films that popular memory has erased almost entirely: The Blockhouse, The Optimists of Nine Elms, and Ghost in the Noonday Sun.

But was this decline, as Thomson suggests, the inevitable result of an incompatibility between Sellers's comedy and the movies as a medium? Or was it rather an archetypal case of great gifts corroded by great personal weakness - in Sellers's case, neurosis, self-distrust and poor professional judgement? The evidence of the Arena biography seems to point to the latter. Far from being bored by the cinema, Sellers was fascinated by it; the first thing he bought when his radio career took off was a cine-camera, and many of the home movies he went on to make were not tender documents of family life but miniature comedies, rather silly but sparkling with his childlike glee at the tricks moving pictures can play.

You can sense the same glee behind his best performances in front of professional cameras. It's easy to note the broadness of his acting, the energy that he always drew from exaggerated cartoonist strokes - somewhere behind the clipped, wizard-prang gaucheness of Captain Mandrake in Dr Strangelove ("I'm a religious man myself, I believe in... all that sort of thing," is his beautifully embarrassed, shuffling response to the hell- fire fervour of General Ripper, who has just triggered global thermo-nuclear war) is the portly ghost of Major Bloodnok from the Goons.

Yet these performances also had more discreet notes. They had been there even in his first screen parts, such as the baby-faced teddy boy Harry Robinson in The Ladykillers (1956), though he learnt the lesson of restraint consciously when working up his role as the stroppy shop steward Fred Kite in I'm All Right, Jack (1959). Sellers was astonished to see the studio technicians convulsing at lines he had initially disliked because he "couldn't find the jokes" - improbable slowness, when you recall that the script includes Kite's immortal one-line summary of the life enjoyed by the happy citizens of the USSR: "all them cornfields, and ballet in the evenings".

True, he tended to be a flashy actor, not to say a bit of a show-off. His real inspiration, though, shows in the details. As Humbert's nemesis Clare Quilty in Stanley Kubrick's Lolita (1962), Sellers pulled off some bravura stunts. His opening scene - it's virtually a soliloquy, conducted in several corny accents - may be the closest that he came to reproducing the wild invention of the Goon Show on film. Yet the moments that make his Quilty unforgettable are much smaller: a slight, narcissistic lift of the eyes, a blas glance down at his watch in mid-step as he struts - a Sixties sophisticate among the hicks - at a high-school dance.

All of Sellers's best roles, save one, have this combination of unrestrained caricature and smaller, more unpredictable detail. Consider some of the quieter moments from his triple role in Dr Strangelove: Captain Mandrake's muttered aside about his wartime tortures at the hands of the Japanese ("strange thing is they make such bloody good cameras"); President Muffley's sudden note of pique when explaining to the Soviet leader that the H-bombs are coming ("Don't say that you're more sorry than I am, because I'm capable of being just as sorry as you are"); or Dr Strangelove's guttural yet oddly soothing tones. As Arena demonstrates, Sellers arrived at that voice by crossing a stage-Nazi accent with the New York speech patterns of Weegee, the famous photographer who was employed by Kubrick as a stills man on the production.

The single exceptional role was almost the last Sellers played. As the simple-minded gardener Chance, or "Chancey Gardiner" in Being There (1979), Sellers finally threw away the tricks, the flourishes and the fantasy, and created a part that was almost nothing but restraint. This was a part he felt very close to, seeing something of his own sense of boyish bewilderment and loss in the holy innocent, and he had lobbied for a decade to bring Jerzy Kosinski's novel to the screen. While there are many ironies to the role of Chance, most of them are happy.

In the Arena films, Jonathan Miller, who directed the actor in the BBC version of Alice in Wonderland (1966), speculates that Sellers's powers as a human chameleon provided him with a sanctuary from "the hideous knowledge that he might be a nonentity". In Being There, Sellers found the courage to shed his comic skills and embrace that nonentity. His renunciation created not just a dignified epitaph for an often messy career, but a character who proved, despite all the actor's misgivings and self-criticisms, to be just as memorable as any of his countless nervous masks.